Oxygen precipitates (BMD; Bulk Micro Defect) in a silicon wafer are useful to capture impurities in a semiconductor device process. The oxygen precipitates are formed in a growing stage of a silicon single crystal, which is a material for a wafer, for example. However, concerning an epitaxial silicon wafer, it is known that the wafer is subjected to a high temperature at a time of epitaxial growth treatment, whereby the oxygen precipitates inside the wafer disappear, and an impurities capturing ability (a gettering ability) is reduced. Consequently, it is desired to provide an epitaxial wafer excellent in impurities capturing ability.
A technique (a pre-annealing technique) has been known (refer to Patent Literature 1, for example) that performs thermal treatment of a wafer at a temperature of 600° C. or more before epitaxial growth treatment, in order to obtain such an epitaxial silicon wafer. By the thermal treatment, the oxygen precipitate density inside the water is increased in advance so that oxygen precipitates remain with a sufficient density after the epitaxial growth treatment, whereby the impurity capturing ability of the wafer after epitaxial growth is enhanced.
Meanwhile, in order to produce a semiconductor device with high quality, it is important to prevent a defect from being introduced into an epitaxial layer of an epitaxial silicon wafer to be a substrate. However, since reduction in a film thickness of the epitaxial layer advances, if a defect is present in a surface layer portion of a silicon wafer on which the epitaxial layer is formed, an epitaxial defect such as a stacking fault due to the defect is likely to be generated in the epitaxial layer.
The defects causing epitaxial defects out of the defects included in a silicon wafer include a dislocation cluster and COP. A dislocation cluster is an aggregate of interstitial silicon atoms that are excessively taken interstitially, and is a defect (a dislocation loop) of a large size of approximately 10 μm, for example. A COP is an aggregate (a vacancy aggregate hollow defect) of vacancies where atoms that should compose a crystal lattice are lost. In order to prevent generation of an epitaxial defect, use of a wafer where neither a dislocation cluster nor COP is present is useful.
As a region where no COP and no dislocation cluster are present in a silicon wafer, there is an oxygen precipitation promotion region (hereinafter, also referred to as “Pv region”) and an oxygen precipitation suppression region (hereinafter, also referred to as “Pi region”). The Pv region is a defect-free region where a vacancy type point defect is dominant, and the Pi region is a defect-free region where an interstitial silicon type point defect is dominant.
When a pulling velocity of a silicon single crystal is V, and a temperature gradient in a growth direction in the single crystal directly after pulling is G, whether a COP appears, a dislocation cluster appears, or neither of them appears depends on V/G. When distances from the center axis of the silicon single crystal are the same, the region including a dislocation cluster, a Pi region, a Pv region and a region including a COP appear sequentially as V/G becomes larger. When a plurality of kinds of regions coexist in a wafer, the respective regions are distributed concentrically with respect to the center of the wafer.
A silicon wafer composed of a region where no COP and no dislocation cluster are present is useful as a substrate wafer for epitaxial growth. However, it is very difficult to grow a silicon single crystal so that an entire region of the wafer is composed of a crystal region of the same kind (for example, only one of the Pv region and the Pi region). This is because a control process margin width, more specifically, a range of allowable V/G is narrow. If it is allowed to grow a single crystal in a range of a growth condition in which both of the Pv region and the Pi region are obtained, the control process margin width increases, and it is possible to produce a crystal that contains no dislocation cluster and no COP stably.